The range of human hearing is usually stated as 20 Hz to 20 kHz. All current audio adapters nominally support this range or close to it. However, few cards state dB for that range, which specifies how flat the frequency response curve is. A good card may have frequency response of 20 Hz to 20 kHz at 3 dB. A professional-level card may have frequency response of 20 Hz to 20 kHz at 1 dB down. In expensive cards may claim frequency response of 20 Hz to 20 kHz, but that range may turn out to be stated at 10 dB down or some similarly absurd number, which in effect means that actual usable frequency response may be something like 100 Hz to 10 kHz.

Signal-to-noise ratio (S/N ratio), stated in dB, specifies the amount of signal (data) relative to noise, with higher numbers indicating better performance. A low S/N ratio translates to audible hiss. The best audio adapters have 95 dB or greater S/N for analog audio; midrange adapters about 90 dB; and inexpensive adapters may have 85 dB or less. It's not unusual for a card to have some what lower S/N ratio for digital recording and digital playback. For example, an excellent consumer-grade audio adapter may specify an S/N ratio of 96 dB FS A-weighted for analog audio, 93 dB FS A-weighted for digital recording, and 90 dB FSA weighted for digital playback. In a typical PC environment, noise level (both ambient external audible noise and the electrically noisy inside of the PC) and the typical use of low-quality speakers or headphones make it unlikely that anyone could differentiate between cards with S/N ratios of 80 dB or higher if that were the only difference. However, cards with higher S/N ratios are generally better shielded and use better components, which translates to better sound and less hiss.

Half-duplex audio adapters can either play sound or capture sound, but not both at the same time. Full-duplex audio adapters do both simultaneously. For simple tasks listening to CDs or playing games a half-duplex card is adequate. More advanced audio functions, such as VoIP telephony and voice recognition, require a full-duplex card. Most midrange and all high-end audio adapters support full-duplex.

In the distant past, software wrote directly to the audio adapter. That meant that compatibility with proprietary standards initially AdLib and later Sound Blaster was important, because if your game or application didn't explicitly support your audio adapter, you simply couldn't use sound with that software. Microsoft took the initiative away from audio adapter manufacturers by incorporating standard sound APIs into Windows. Here are the standards you should be aware of:

Microsoft DirectSound (DS) is a component of DirectX. Developers can write to the DS API, rather than to the underlying hardware, with the assurance that their software will function with any DS compatible audio adapter. DS compatibility is an absolute requirement for any audio adapter.

Microsoft DirectSound3D (DS3D) is an extension to DS that supports 3D positional audio, which is a technology that manipulates sound information to extend stereo imaging to full surround sound, allowing sounds to appear to come from any position around you. For example, when in an air combat game your missile hits a bandit in front of you, the sound of that explosion comes from the front. But if you didn't notice his wingman on your six, the sound of his missile blowing off your tail comes from behind. The realism of DS3D imaging in any given situation depends on how many speakers you use and the hardware capabilities of the audio adapter. If you intend to use DS3D-enabled software, it's important to have hardware support for DS3D in your audio adapter, because DS3D positional effects that cannot be processed in hardware are processed by the main CPU, which can bog down system performance.

Midrange and high-end audio adapters have an on board DSP, which is a general-purpose CPU optimized for processing digital signals, such as audio. In 2D mode, the DSP provides enhanced audio effects like chorus, reverb, and distortion. In 3D mode, it processes 3D-positional audio (e.g., DirectSound 3D or EAX) algorithms locally, removing that burden from the main CPU. Inexpensive audio adapters use the host CPU, which reduces performance significantly, particularly during complex operations such as 3D rendering. Any accelerated audio adapter should accelerate 32 or more DS and DS 3D sound streams in hardware.

External USB Sound Adapters

Several companies, including Creative Labs, M-Audio, and Turtle Beach, manufacture external audio adapters that connect to a PC via a USB port. The advantage of these devices is easy installation you just connect the box to a USB port and install the drivers; no need to open the case. The disadvantage is that you have one more box cluttering up your desk.